Hypoxia upregulates hypoxia inducible factor (HIF)-3α expression in lung epithelial cells:: characterization and comparison with HIF-1α

The role of the hypoxia-inducible factor (HIF) subunits 1 alpha and 2 alpha in response to hypoxia. is well established in lung epithelial cells, whereas little is known about HIF-3 alpha with respect to transcriptional and translational regulation by hypoxia. HIF-3 alpha and HIF-1 alpha are two similar but distinct basic helix-loop-helix-PAS proteins, which have been postulated to activate hypoxia responsive genes in response to hypoxia. Here, we used quantitative real time RT-PCR and immunoblotting to determine the activation of HIF-3 alpha vs. HIF-1 alpha by hypoxia. HIF-3 alpha was strongly induced by hypoxia (1% O-2) both at the level of protein and mRNA due to an increase in protein stability and transcriptional activation, whereas HIF-1 alpha protein and mRNA levels enhanced transiently and then decreased because of a reduction in its mRNA stability in A549 cells, as measured on mRNA and protein levels. Interestingly, HIF-3 alpha and HIF-1 alpha exhibited strikingly similar responses to a variety of activating or inhibitory pharmacological agents. These results demonstrate that HIF-3 alpha is expressed abundantly in lung epithelial cells, and that the transcriptional induction of HIF-3 alpha plays an important role in the response to hypoxia in vitro. Our findings suggest that HIF-3 alpha, as a member of the HIF system, is complementary rather than redundant to HIF-1 alpha induction in protection against hypoxic damage, in alveolar epithelial cells.